1. Technical Field
The present invention relates in general to improved calibration of glide heads and, in particular, to an improved system, method, and apparatus for measuring media roughness and combining that with spin down calibration to form a new and improved glide head calibration for media glide testing.
2. Description of the Related Art
Data access and storage systems comprise one or more storage devices that store data on magnetic or optical storage media. For example, a magnetic storage device is known as a direct access storage device (DASD) or a hard disk drive (HDD) and includes one or more disks and a disk controller to manage local operations concerning the disks. The hard disks themselves are usually made of aluminum alloy or a mixture of glass and ceramic, and are covered with a magnetic coating. Typically, two or three disks are stacked vertically on a common spindle that is turned by a disk drive motor at several thousand revolutions per minute (rpm).
The HDD also has an actuator assembly with an actuator that moves magnetic read/write heads to the desired location on the rotating disk so as to write information to or read data from that location. Within most HDDs, the magnetic read/write head is mounted on a slider. A slider generally serves to mechanically support the head and any electrical connections between the head and the rest of the disk drive system. The slider is aerodynamically shaped to glide over moving air in order to maintain a uniform distance from the surface of the rotating disk, thereby preventing the head from undesirably contacting the disk.
Typically, a slider is formed with an aerodynamic pattern of protrusions or air bearing design on its air bearing surface (ABS) that enables the slider to fly at a constant height close to the disk during operation of the disk drive. A slider is associated with each side of each platter and flies just over the platter's surface. Each slider is mounted on a suspension to form a head gimbal assembly (HGA). The HGA is then attached to a semi-rigid actuator arm that supports the entire head flying unit. Several semi-rigid arms may be combined to form a single movable unit having either a linear bearing or a rotary pivotal bearing system.
The head and arm assembly is linearly or pivotally moved utilizing a magnet/coil structure that is often called a voice coil motor (VCM). The stator of a VCM is mounted to a base plate or casting on which the spindle is also mounted. The base casting with its spindle, actuator VCM, and internal filtration system is then enclosed with a cover and seal assembly to ensure that no contaminants can enter and adversely affect the reliability of the slider flying over the disk. When current is fed to the motor, the VCM develops force or torque that is substantially proportional to the applied current. The arm acceleration is therefore substantially proportional to the magnitude of the current. As the read/write head approaches a desired track, a reverse polarity signal is applied to the actuator, causing the signal to act as a brake, and ideally causing the read/write head to stop directly over the desired track.
The presence of asperities on the surfaces of the disks can have a deleterious effect on the performance of disk drives. For this reason, a glide test is performed on finished disks to detect asperities that might contact the magnetic head flying at its normal height in a disk drive. In the test, a special glide head containing a piezoelectric transducer (PZT) is flown over a disk at an altitude or height that is below the normal drive fly height. Glide head contact with an asperity creates a PZT voltage response that generally scales with increasing size of the asperity. If the voltage response exceeds a predetermined level, the disk is rejected. As such, quantitative glide testing requires calibration of the voltage response with respect to asperity height.
In the prior art, there are a number of methods for calibrating glide head fly heights. One method of calibrating glide heads uses a laser-generated, nano-sized protrusion or bump on the surface of a disk that can serve as a calibration asperity. Laser nano-bump generation is a technique that is used throughout the data storage industry. Flying a glide head over a laser nano-bump whose height is known (e.g., by interference or atomic force microscopy) will thus generate a calibrated PZT response. However, when using a single laser calibration bump, statistical variation in the PZT signal from one run to another results in a wide response distribution.
In another method, the glide head is spun down onto bumps with a known average height arrayed around one or more tracks on a disk media. When the glide head touches the top of the bumps, that linear velocity is used to set the glide test at the average height of the bumps. This “spin down to bump” calibration methodology can only support bump heights down to about 8 nm. The laser melt process used to generate the bump disk media has difficulties maintaining bump height accuracies below 8 nm. However, present and future performance requirements need glide head fly heights down to about 4 nm.
One solution to this problem is to spin down onto the disk media with a substrate roughness that is very near the desired low fly height. Fortunately, most glass substrate roughness is in the range of about 3 to 4 nm. Substrate texturing techniques can generate spin down response up to about 7 nm. However, this solution alone has some deficiencies. A good glide head calibration technique should do more than just (a) set the right fly height for glide testing. The technique should also check each glide head for (b) proper flying characteristics and (c) uniform PZT response. Thus, since parameters (b) and (c) are not provided, the glide head PZT signal generated by spinning down onto the media roughness does not hold enough information to satisfy the requirements of a good calibration technique.
A second problem with spinning down onto disk media roughness is that the current glide PZT channel has problems handling the PZT response of the glide head flying at the media roughness height. Typical glide channels were designed to analyze glide heads hitting discreet media defects producing PZT signals with very fast rise and decay times. These transitions in the PZT signal trigger software interrupts allowing each discreet hit to be counted. The PZT signal of a glide head on media roughness can resemble a DC phenomena shift from low to high and stay high due to the many contacts around the track. When this happens, one cannot determine if the glide head is hitting one discreet defect or many contacts due to the media roughness.
The PZT RMS signal could be used to measure the disk media roughness with spin down. However, one significant weakness of RMS is that a single defect with sufficient amplitude can greatly affect the RMS reading of the entire track and thereby render useless any information gleaned therefrom. Thus, an improved system, method, and apparatus for glide head calibration at lower fly heights would be desirable.